This is unlikely to be due to a change in Chrome, as UMA shows that the error rate spikes at the same date no matter the Chrome version.

It is likely due to large response bodies, as the limit exludes response headers.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0c874e17e4f9429718c1e79f1a80fd13ea0cf8a7

commit 0c874e17e4f9429718c1e79f1a80fd13ea0cf8a7
Author: Benoît Lizé <lizeb@chromium.org>
Date: Mon Nov 19 13:00:41 2018

customtabs: Add large response tests for DetachedResourceRequest.

Large headers should not be counted, only large response bodies.

Bug:  906042 
Change-Id: I941376f671ad63500e3676811ab74c80e659c0fa
Reviewed-on: https://chromium-review.googlesource.com/c/1340310
Reviewed-by: Alexandr Ilin <alexilin@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609256}
[modify] https://crrev.com/0c874e17e4f9429718c1e79f1a80fd13ea0cf8a7/chrome/browser/android/customtabs/detached_resource_request.cc
[modify] https://crrev.com/0c874e17e4f9429718c1e79f1a80fd13ea0cf8a7/chrome/browser/android/customtabs/detached_resource_request.h
[modify] https://crrev.com/0c874e17e4f9429718c1e79f1a80fd13ea0cf8a7/chrome/browser/android/customtabs/detached_resource_request_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/11d41bdc5979edbc9b301598e4dbf183492ddc28

commit 11d41bdc5979edbc9b301598e4dbf183492ddc28
Author: Benoît Lizé <lizeb@chromium.org>
Date: Tue Nov 20 09:29:57 2018

customtabs: Check that detached requests set cookies for truncated responses.

Bug:  906042 
Change-Id: Ic65a91052397be9a2b0facbb9924114edd7904a2
Reviewed-on: https://chromium-review.googlesource.com/c/1341839
Reviewed-by: Alexandr Ilin <alexilin@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609642}
[modify] https://crrev.com/11d41bdc5979edbc9b301598e4dbf183492ddc28/chrome/browser/android/customtabs/detached_resource_request_unittest.cc

Summary:
This comes from a CCT client changing its behavior, and sometimes responding with a HTTP code 200 instead of 204. Added tests to check that the desired behavior is and remains the one we have, and as a follow-up will discard the body from 200 responses.

Closing the bug as everything is WAI.